Support multiple client certificates with per-request selection

**Is your feature request related to a problem? Please describe.**
Right now you can only store one client certificate per collection. If you need to send requests to different servers that each require a different certificate, you have to manually replace the certificate in Collection Settings every time — which is tedious and easy to forget.

**Describe the solution you'd like**
**Collection Settings – mTLS tab:**
The mTLS tab gets a list view similar to the Environments tab. You can press + to add a new certificate, give it a name (e.g. "Production", "Staging"), and fill in the cert/key/ca files. Multiple certificates can be stored at the same time.

**Per-request – new TLS tab:**
Each request gets a new TLS tab. Inside that tab there is a simple dropdown that shows all the certificates you added in Collection Settings. You pick the one you want to use for that specific request. If you don't pick one, no certificate is sent — same as today.

**Technical Notes:**
The collection needs to store multiple named certificates instead of just one (requires a schema migration)
Each request needs to remember which certificate was selected
When sending a request, the backend looks up the selected certificate and uses it

**Additional context**
The base implementation (#620) already handles sending a single client certificate per request. This issue builds on top of that by allowing multiple certificates to be stored and selected individually per request.